Motivated by the desire to understand the structure-property relationships of biomolecules including DNA, RNA and membranes and the roles they play in life processes, chemists have striven to manipulate molecular-scale phenomena, resulting from noncovalent bonding interactions, in ever-increasingly complex and organized situations.[1-4] By employing noncovalent bonding interactions, synthetic foldamers,[5-8] which are promising candidates for mimicking the behavior of biomacromolecules under different kinds of stimuli—and mechanical interlocked molecules[9-11] (MIMs), which are the result of the formation of mechanical bonds and have already found applications in drug delivery[12, 13] and molecular electronics[14-17]—have been developed and investigated in some detail. Foldamers and MIMs, both utilizing intra- and intermolecular interactions in order to regulate the shapes of molecules, however, seldom result in their paths' crossing.
Foldameric rotaxanes,[18-21] which lie at the intersection between synthetic foldamers and MIMs, have made their ways into chemists' sights recently. Usually expressed in the context of oligorotaxanes, in which the dumbbell component is threaded by multiple ring components in order to regulate the folded secondary structure, they can exhibit remarkable physicochemical properties[22-26] in response to external stimuli. For example, it has already been[22, 23, 25] demonstrated that mechanical responses of oligorotaxanes toward external forces can be controlled by the mobile rings trapped along their one-dimensional dumbbell components, representing a new class of entropy-dominated molecules and materials.
To date, syntheses and properties of a family of foldameric oligorotaxanes which rely on the presence of donor-acceptor recognition between electron-rich 1,5-dioxanaphthalene (DNP) units and electron-deficient cyclobis(paraquat-p-phenylene) (CBPQT4+) rings have been disclosed.[28] Moreover radical-pairing interactions associated with BIPY(•+) radical cations—the mono reduced state of dicationic BIPY2+ units—can be utilized in the preparation of MIMs based on a template-directing strategy.[29-33]
Foldameric oligorotaxanes make it possible to prepare functional materials by scaling[11,27] the concerted mechanical actuation of MIMs into the macroscopic regime where applications can be sought and witnessed. For the development of new applications and novel molecular devices and materials, there is a need for new rotaxanes that can be used to prepare nanoactuators.